The present invention relates to optical devices and, more particularly, to an optical device that rotates with respect to an incident beam of light in such a way that the exiting beam remains substantially stationary as the device rotates.
U.S. Pat. No. 5,539,517 to Cabib et al., which is incorporated by reference for all purposes as if fully set forth herein, teaches a variety of spectral imaging devices based on interferometers. Among these interferometers is the Sagnac interferometer illustrated in FIG. 6 of that patent. As explained in the text of that patent with reference to FIGS. 7 and 8, this Sagnac interferometer can be used in two different modes. FIG. 7 illustrates a non-moving mode, in which the optical path difference within the interferometer is scanned by scanning the angle with which the incident beam enters the interferometer. FIG. 8 illustrates a moving mode, in which the optical path difference is scanned by rotating the interferometer as a whole with respect to the incident beam.
To maximize the spectral resolution of the systems of Cabib et al., it is important that the range of optical path differences scanned be as large as possible. The major limiting factor in the optical path difference range of a Sagnac interferometer is "vignetting". As the interferometer is rotated with respect to an incident beam, the exiting beam also moves. There is thus a widely recognized need for, and it would be highly advantageous to have, a Sagnac interferometer having minimal motion of the exiting beam as the interferometer is rotated with respect to the incident beam to scan optical path differences.